1. Field of the Invention
This invention relates to the field of adjusting the resonant frequency of piezoelectric resonators and more particularly to procedures for tuning piezoelectric resonators after the resonator has been packaged in a hermetically sealed enclosure.
2. Description of the Prior Art
Many methods of tuning piezoelectric resonators prior to packaging in a sealed enclosure are well known in the art. Such methods include mass loading by sputtering, mass removal by etching, spot plating by metal deposition, dielectric mass loading, mass removal by laser trimming, chemical reaction from gas atmosphere, direct plating to frequency, and mass loading by electroplating are well known and frequently used. Of these methods only one, dielectric mass loading, has been shown to be adaptable to tuning resonators after packaging. This is especially true if the packaging enclosure is hermetically sealed. It is frequently necessary to tune after packaging to avoid disturbing the resonator frequency during the sealing process and to adjust resonators in the field to counteract aging effects.
In the prior art in-can tuning method of dielectric mass loading, the resonator is placed within a specialized can which has been back-filled with a polymerizable gas such as tetrafluoroethylene. The gas is polymerized by radiation and the resulting polymer deposits on the resonator increasing its mass and lowering its resonant frequency. Once the polymerizable gas has been used up no further adjustment is possible. Therefore, it is evident this method cannot be used more than once for any given device and the process is not reversible to correct overadjustment.
A major disadvantage of this prior art process is that a specialized enclosure must be used resulting in greater expense. Preferably, the enclosure must be made of a material which is transparent to the radiation being used for the polymerization. The preferred enclosure material is fused quartz or sapphire since these materials allow some of the less dangerous forms of radiation to be used to polymerize the gas within the enclosure. Alternatively, the housing may contain a source of radiation itself such as a glow discharge source but this also increases the cost of the enclosure.
The recommended enclosure for this prior art process also includes a separate compartment shielded from the main portion of the enclosure. Any remaining unpolymerized gas is polymerized in this compartment after the trimming or adjustment is complete. The unpolymerized gas must be eliminated after the trim in order to avoid excessive aging and thermal hysteresis problems. This process is, therefore, costly, difficult to implement and non-reversible.
A further disadvantage of this prior art method is that mass loading of resonators with dielectric material can reduce the Q and raise the series resistance of the resonator. This is especially critical in higher frequency resonators, and severely limits the electrical performance of VHF crystals so processed. There is also some concern for the long term stability of crystals which are trimmed according to this process.